[Pre-RFC] Inferred Enum Type

[scottmcm]

@jkugelman

If this feature were to be added, I actually prefer @scottmcm's .Variant syntax. .Item is easier on the eyes than _::Item .

I think I first mentioned this syntax in Auto infer namespaces on struct and enum instantiations - #6 by scottmcm, but all I did is shamelessly steal it from Swift https://docs.swift.org/swift-book/LanguageGuide/Enumerations.html#ID147.